Recent research conducted by Carol Folt and her colleagues in the department of biological sciences at Dartmouth discusses on the relationship between mercury concentration and the growth rate of stream-dwelling Atlantic salmon.

Mercury (Hg) contamination in fish has potentially dangerous health risks because of widespread fresh consumption, which is the primary source of exposure to toxic methylmercury (MeHg). Given the importance of reducing Hg contamination, it is essential to identify risk populations and re-evaluate management practices that could be worsening the accumulation of Hg in fisheries.

Prior research in freshwater lakes identified a few factors that could promote Hg accumulation, but most of the research has not specifically focused on the stream fish themselves. Also, most research on metal contamination in stream food has focused on sites affected by point sources.

Folt hypothesized that manipulation of fish growth might possibly lead to the management of Hg contamination in fisheries. The researchers examined the relationships between the Hg concentration of the fish, the Hg concentration of their prey, and individual growth and size of the fish.

Folt created a large-scale field experiment in natural streams. The field was controlled–there was no Hg near the selected streams. The team picked 18 study sites located on six small tributary streams and stocked the streams with juvenile Atlantic salmon that are not indigenous to the region. The fish, after the maturation period, were euthanatized and processed for the presence of Hg. The research team removed the stomach contents and processed the foreguts separately to estimate the Hg concentration of invertebrate prey. Prior studies have shown that stomach contents can disclose metal concentrations in the prey of the fish.

There were two predictors of Hg concentration of particular interest: the growth of the individual salmon (measured as final mass), and the Hg concentration in the prey. As expected, the Hg concentration in the prey was the most important predictor of the concentration in the fish. The larger, faster-growing fish had lower Hg concentrations.

Given the fact that high stocking density leads to suppressed fish growth in many fisheries, this study is highly relevant. Folt suggests that intensive fishing can reduce the fish population density, which in turn can lead to higher growth rates.

Folt hopes to continue work in this topic. According to researcher Darren Ward, “Right now we’re doing some laboratory experiments to evaluate the effects of food quality on fish growth and mercury accumulation…We’re also applying out mercury work to more basic ecological questions- measuring mercury accumulation through food webs can be a powerful tool for tracing the flow of energy in an ecosystem.”